The invention relates to a measuring apparatus with at least one photoelectric transducer for the determination of the color composition of the copying light in photographic enlarging apparatus.
Known measuring apparatus of such type have at least one measuring probe, containing a photoelectric transducer, which is inserted into the copying light beam path directly below the lens of an enlarging apparatus during the measuring procedure. Such a measuring position of the probe is selected, inter alia, because a measuring light of greater intensity is available in this position, whereby relatively strong measuring signals may be obtained and relatively inexpensive photoelectric transducers and/or circuits may be employed for the evaluation of such signals.
At the outlet of the lens, the light beam contains differently distributed, color-distinguished zones, according to the color distribution over the cross-sectional area. The colors are, therefore, not mixed in such a manner that an overall measurement, entirely free of error is possible. If only one transducer is utilized, care must be taken that the measuring probe is disposed in the same position relative to the lens for each measurement, because even negligible displacements can affect the measuring result accuracy. This is because, as previously mentioned, the light impinging on the transducer is not homogeneous in color content and consequently with displacement from positions previously taken, a different color component is measured. If several transducers are inserted into the light beam simultaneously in order to have simultaneously available a measuring signal for two or all three primary colors, for example, consistent measuring results may be achieved only then when the measuring light is mixed before it impinges on the transducers, something which requires, therefor, a light-mixing compartment of a certain length or other known means for light mixing. When utilizing such means, the light available for the measuring is diminished. Furthermore, measuring probes with light-mixing means are larger and heavier, for which reason their insertion and eventual mounting below the lens is cumbersome. Still further when light-mixing means is used, it is impossible to construct the measuring arrangement as small as otherwise afforded by the known circuits for light measuring apparatus.
For measuring, the probe may be mounted either below the lens, e.g., at the housing thereof, or it may be pivoted into this position. In the first solution, the operation is cumbersome, because the measuring probe must be individually positioned for each measurement and must repeatedly be removed and set aside during each copying operation. In the second-mentioned procedure, the measuring probe pivots about an axis on the enlarging apparatus, e.g., on the lens holder. In order that a pivoting of the measuring probe below the lens may be possible for various types of apparatus and lenses, the rotary mounting must be constructed relatively expensively or there must be produced an individual mounting device for each apparatus-lens combination, which in both instances unduly increase the cost of the measuring apparatus.
The object of this invention is to provide a measuring apparatus of the initially described type, wherein precise positioning of the photoelectric transducer has less of an influence on the color measuring accuracy than with the known apparatus wherein even in the use of several photoelectric transducers, light-mixing means are not necessary and wherein the other mentioned disadvantages are eliminated.